Robert Forward, RIP, died in 2002 age 70.
One of his ideas he patented was about the possibility of a satellite about 1 million miles above the north or south pole, held in place permanently by solar sails, no fuel needed, and it could monitor half the world in one go, from the north pole, see anything launched by Russia or China or Korea and get that information back to Earth in 10 seconds or so.
It would not be too useful as a cell phone relay because of the 5 second or so delay of RF going the speed of light, and twice that for round trip but for a weather sat or military monitor, 5 second delay would mean nothing.
I thought it was a really clever idea.
He wrote 11 sci fi books and published over 200 science papers. A really smart dude.
This piece is from New Scientist, CA 1991:
https://www.newscientist.com/article/mg12917594-000-science-polar-satellite-could-revolutionisecommunications/
Crunching the numbers, at an altitude of 1 million miles the gravitational attraction of Earth would be about 1/62000ths of surface gravity, about 150 MICRONS/sec^2 acceleration or less so it wouldn't need a whole lot of solar sail thrust to keep it in a stable 'orbit'.
@sonhouse
He is probably referring to Lagrange points. I believe there are 5 of them where net gravity from Earth and Sun balance out to zero and maintain a constant distance from Earth.
@bunnyknight
Nope, he is talking about a place about a million miles from Earth not L2 etc, but held in place strictly by solar sails, 24/7 providing thrust needed to keep the object at the right altitude.
http://www.esa.int/Science_Exploration/Space_Science/Herschel/L2_the_second_Lagrangian_Point
Notice, those points lie around the line between Earth and sun, not up in the air a million miles or so.
This object is independent of Lagrange points.
That object would have solar panels complimenting the sails, maybe even part of the sails and provide energy for transponders and telescopes and such. Of course if it died, there would be no trip up there with humans to fix things so they would probably have triple and quadruple redundancy equipment onboard.
@sonhouse
OK, I see what you mean. But how would you prevent the north satellite from flying off into space during winter, as the solar pressure only adds to the outward push?
@bunnyknight
Just the angle of the sails, just like sails on watercraft, you can tack into the wind and so forth.
@sonhouse
It may not be possible to steer straight back into solar wind in the vacuum of space. However, with the aid of electromagnetic propulsion it might work.
@bunnyknight
It is tricky but possible but I don't think that would ever be needed. It would be actually at the 23.5 degree angle of tilt of the planet so it would hang around not at the pole lines but offset by that amount which gives the sails total access to the sun.